The Transformation Brooch was the first item that Serena Tsukino used to transform into Sailor Moon. It's transformation is Moon Prism Power, Make Up!.
It was given to Usagi by Luna in episode 1 of the anime and Act 1 of the manga. Luna bribed her into transforming into Sailor Moon order to prove to Serena that she had a destiny as a Sailor Scout. Once she obtained it, Serena began wearing the brooch in the center of the bow on the front of her school uniform when she was in civilian form.
The brooch was broken in episode 51 of the anime and Act 14 of the manga, but received the Crystal Star afterwards. Starting with the Crystal Star, all of Serena's other transformation brooches were upgrades of previous brooches.